The present invention relates generally to weighing apparatuses for powder. More particularly, the present invention relates to vibrator and screw combined conveying devices for the weighing apparatus for finely ground powders, for example pharmaceutical powders, in which a vibrator is incorporated into the conveying device to provide a vibrational conveying action, irrespective of the normal conveying operation by a driving shaft having spiral blades thereon, so that a wide range of weight of powder, particularly a very small quantity of powder, can be exited from the conveying device of the invention.
Prior art conveying device used in weighing apparatuses are not suitable for exiting small quantities of powder by a rotational action of a shaft with helical blades, not only because the number of rotations of the shaft is generally difficult to finely control but also because the quantity of powder that will be exited in one rotation is difficult to calculate or control. As is well known the shaft with helical blades, or a screw as is generally called, like an auger in outer appearance, is generally provided within a hollow cylinder for exiting powder by means of the rotational movement of the shaft about its own axis. Under a given rotation speed of the shaft, the quantity of powder exited is proportional to the peripheral diameter of the blades. To exit different quantities of powder, two or more sets of cylinders and augers of different size may be required. For high precision measurements, for example, in the order of 0.01 g, it is necessary to provide a cylinder and an auger both of a very small size. However, this still does not resolve the problem that the quantity of powder to be exited in one rotation of the shaft is difficult to anticipate, such as a spontaneous falldown of a pile of powder.
Another problem encountered in prior art conveying devices is that "viscous" powder, as well as coarse powder, that might be harmful to the blades and the cylinder wall, tends to cohere or coagulate, thereby interfering with the rotation of the shaft. In other words, pushing force of the blades by a simple rotational movement of the shaft is not sufficient to prevent such powders from causing damage to the conveying device.
There is a need for the conveyed quantity of powder to be finely controlled to meet highly precise tolerances, as well as a need for satisfying wide range of quantities that can be measured in a single device to suit practical use. Also, such a conveying device should be able to handle "viscous" and very coarse powders in an effective manner as for ordinary powder.